(1) Field of the Invention
This invention generally relates to antennas and more specifically to quadrifilar antennas.
(2) Description of the Prior Art
Numerous communication networks utilize omnidirectional antenna systems to establish communications between various stations in the network. In some networks one or more stations may be mobile while others may be fixed land-based or satellite stations. Antenna systems that are omnidirectional in a horizontal plane are preferred in such applications because alternative highly directional antenna systems become difficult to apply, particularly at a mobile station that may communicate with both fixed land-based and satellite stations. In such applications it is desirable to provide a horizontally omnidirectional antenna system that is compact yet characterized by a wide bandwidth and a good front-to-back ratio in elevation with circular polarization for satellite communications.
Some prior art omnidirectional antenna systems use an end fed quadrifilar helix antenna for satellite communication and a co-mounted dipole antenna for land based communications. However, each antenna has a limited bandwidth. Collectively their performance can be dependent upon antenna position relative to a ground plane. The dipole antenna has no front-to-back ratio and thus its performance can be severely degraded by heavy reflections when the antenna is mounted on a ship, particularly over low elevation angles. These co-mounted antennas also have spatial requirements that can limit their use in confined areas aboard ships or similar mobile stations.
The following patents disclose helical antennas that exhibit some, but not all, of the previously described desirable characteristics:
U.S. Pat. No. 5,329,287 (1994) Strickland et al.
U.S. Pat. No. 5,489,916 (1996) Waterman et al.
U.S. Pat. No. 5,572,227 (1996) Pal et al.
U.S. Pat. No. 5,604,972 (1997) McCarrick
U.S. Pat. No. 5,612,707 (1997) Vaughn et al.
U.S. Pat. No. 5,329,287 to Strickland discloses a device for use in a helical antenna having an antenna element wound about the periphery of a tubular or cylindrical dielectric support post. The device has an electrically conductive member electrically connected to one end of the antenna element. The conductive member is of any appropriate shape or configuration and is operable to increase the loading on the antenna whereby standing waves on the antenna element are reduced and a more uniform electrical current is produced along the antenna element.
U.S. Pat. No. 5,498,916 to Waterman discloses a quadrifilar helical antenna including four conductive helices having a common central axis, a common direction of turn about said axis, a common pitch and a common length between opposite ends. The helices are uniformly spaced from each other by 90xc2x0, with a single dielectric helix concentric with the common axis, lying within and supporting the conductive helices at a nominal diameter. The dielectric helix has opposite ends, a plurality of turns having said common direction of turn, and a second pitch substantially greater than said common pitch. A casing contains the helices and is rotatably fixed to one end of the dielectric helix. A tuning device is fixed to the other end of the dielectric helix and rotatable relative to said casing, so that rotation of the tuning device twists the dielectric helix to alter the common pitch of the conductive helices and thus the elevation patterns of the antenna, without substantial variation from said nominal diameter.
U.S. Pat. No. 5,572,227 to Pal et al. discloses a multiband antenna system for operating at L-band, S-band and UHF-band frequencies. The antenna includes L-band antenna elements and S-band antenna elements provided in the form of quadrifilar helices spaced from each other on the surface of a hollow cylindrical insulator. UHF band antenna elements are provided in the form of a cage dipole on the surface of the hollow cylindrical insulator. The L-band antenna input is connected to a first connector through an L-band feed network card. The S-band antenna input is connected to a second connector through an S-band feed network card and the UHF-band antenna input is connected to a third connector through a split sheath balun provided along the axis of the hollow cylindrical insulator.
U.S. Pat. No. 5,604,972 to McCarrick discloses a mobile vehicular antenna for use in accessing stationary geosynchronous and/or geostable satellites. A multi-turn quadrifilar helix antenna is fed in phase rotation at its base and is provided with a pitch and/or diameter adjustment for the helix elements, causing beam scanning in the elevation plane while remaining relatively omnidirectional in azimuth. The antenna diameter and helical pitch are optimized to reduce the frequency scanning effect. A technique is provided for aiming the antenna to compensate for any remaining frequency scanning effect.
U.S. Pat. No. 5,612,707 to Vaughn et al. discloses a variable helix antenna consisting of one or more conductors affixed to a furled dielectric sheet. The antenna beam is steerable by furling and unfurling of the dielectric sheet either rotationally, axially or by a combination of both. Multiple interleaved dielectric sheets may be used for multifilar embodiments and matching and compensation elements may also be provided on the dielectric sheet.
In addition to the foregoing antennas, there exists a family of quadrifilar helixes that are broadband impedance wise above a certain xe2x80x9ccut-inxe2x80x9d frequency, and thus are useful for wideband satellite communications including SATCOM (Satellite Communications) and Demand Assigned Multiple Access (DAMA) UHF functions in the range of 240 to 320 MHz and for other satellite communications functions in the range of 320 to 410 MHz. For example, my above-identified pending U.S. patent application Ser. No. 09/356,808 discloses an antenna having four constant-width antenna elements wrapped about the periphery of a cylindrical support. This construction provides a broadband antenna with a bandwidth of 240 to at least 400 MHz and with an input impedance in a normal range, e.g., 100 ohms. This antenna also exhibits a good front-to-back ratio in both open-ended and shorted configurations. In this antenna, each antenna element has a width corresponding to about 95% of the available width for that element.
Typically these antennas have (1) a pitch angle of the elements on the helix cylindrical surface from 50xc2x0 down to roughly 20xc2x0, (2) elements that are at least roughly xc2xe wavelengths long, and (3) a xe2x80x9ccut-inxe2x80x9d frequency roughly corresponding to a frequency at which a wavelength is twice the length of one turn of the antenna element. This dependence changes with pitch angle. Above the xe2x80x9ccut-inxe2x80x9d frequency, the helix has an approximately flat VSWR around 2:1 or less (about the Zo value of the antenna). Thus the antenna is broadband impedance-wise above the cut-in frequency. The previous three dimensions translate into a helix diameter of 0.1 to 0.2 wavelengths at the cut-in frequency.
For pitch angles of approximately 30xc2x0 to 50xc2x0, such antennas provide good cardioid shaped patterns for satellite communications. Good circular polarization exists down to the horizon since the antenna is greater than 1.5 wavelengths long (2 elements constitute one array of the dual array, quadrifilar antenna) and is at least one turn. At the cut-in frequency, lower angled helixes have sharper patterns. As frequency increases, patterns start to flatten overhead and spread out near the horizon. For a given satellite band to be covered, a tradeoff can be chosen on how sharp the pattern is allowed to be at the bottom of the band and how much it can be spread out by the time the top of the band is reached. This tradeoff is made by choosing where the band should start relative to the cut-in frequency and the pitch angle.
For optimum front-to-back ratio performance, the bottom of the band should start at the cut-in frequency. This is because, for a given element thickness, backside radiation increases with frequency (the front-to-back ratio decreases with frequency). This decrease of front-to-back ratio with frequency limits the antenna immunity to multipath nulling effects.
Therefore it is an object of this invention to provide a broadband unidirectional hemispherical coverage antenna.
Another object of this invention is to provide a broadband unidirectional hemispherical coverage antenna with good front-to-back ratio.
Yet another object of this invention is to provide a broadband unidirectional hemispherical coverage antenna that operates with circular polarization.
Yet still another object of this invention is to provide a broadband unidirectional hemispherical coverage antenna that operates with a circular polarization and that exhibits a good front-to-back ratio.
Yet still another object of this invention is to provide a broadband unidirectional hemispherical coverage antenna that is simple to construct and is lightweight.
A further object of this invention is to provide a broadband unidirectional hemispherical coverage antenna having low and high angle patterns.
An antenna constructed in accordance with one aspect of this invention connects to an rf source and includes a plurality of antenna elements. Each antenna element has a helical form extending along an antenna axis and is spaced from others of said antenna elements. Each antenna element has a first pitch angle relative to a plane normal to the antenna axis at a first end and a greater pitch angle at its second end. Rf feed points can be selected at either end of said antenna. The connection of the rf source to the selected rf feed points at one end of said antenna determines the operation said antenna as a low-angle or high-angle antenna.